Recently impressive results have been obtained with EPR studies of living animals and perfused organs, using low frequency EPR. In many instances in such studies, however, there are apparent distortions of the spectra. The shapes of thesespectra and theoretical considerations indicate that these effects are due to e ddy currents in the moderately conductive biological materials. Therefore we have systematically studied the effects of eddy currents produced in biological samples under typical conditions being employed for in vivo EPR studies in order to determine the extent of these effects and to develop methods to compensatefor these effects. The presence of eddy currents was found to decrease the Q f actor and distort the shapes of the EPR spectra. The distortion of the spectra led to line width broadening, changes in peak heights, and shifts of the apparent center of the lines. These effects could be corrected instrumentally, but this was effective only when signals have a high S/N. The use of appropriate computer simulations, based on linear combinations of currents and provide the accurate data on the parameters of EPR spectra that is needed for most or all of the analyses used in biological studies of animals and tissues at low frequencies.